1. Field of the Invention
The present invention relates to a metal halide discharge lamp, and more particularly, to a metal halide discharge lamp using tin as a metal emitting visible light.
2. Description of the Related Art
There have been known many kinds of metal halide lamps using at least one kind of metal emitting visible light. It is known to build a small sized single ended metal halide lamp. It has a pair of electrodes positioned at one side of a light emitting tube. It has a high wall load of not less than 25 W/cm.sup.2 for emitting a high intensity light. The wall load is much greater than those of metal halide lamps of the type which have a pair of electrodes which are located at respective opposite ends of the light emitting tube which have a wall load of 10 to 18 W/cm.sup.2.
The metal halide lamp having a high wall load can not use rare earth metals, such as dysprosium, which have high efficiency in emitting light and have high color rendition of light. When a lamp has a high wall load, the temperature of the light emitting tube increases and dysprosium is likely to react with quartz which is a material of the light emitting tube at high temperature. When dysprosium reacts with the light emitting tube made of quartz, the light emitting tube can not transmit light and dysprosium decreases. Therefore, metal halide lamps having a high wall load use other metals for emitting visible light such as thallium, sodium, tin, mercury and so on, as shown in U.S. Pat. No. 4,717,852. Tin emits visible light having high color rendition. Thallium and sodium are used for improving the efficiency and the color rendition of light.
However, the metal halide lamp not using dysprosium has a correlative color temperature (which will be called "clcolor temperature" in the below) of about 3000.degree. K. It is too low, as compared with a color temperature of 4000 to 5000.degree. K. which the metal halide lamp has by using dysprosium.
It is known to use metals emitting more blue or green light for increasing a color temperature of a metal halide lamp having a low wall load. However, when the same technology is applied to a metal halide lamp having a high wall load, it does not overcome the problem of decreased efficiency or color rendition because of difference in wall load.